The invention concerns a superconducting magnet coil system with a main field coil which is formed from radially nested, series-connected sections comprising conductors, wherein the conductors of the radially inner sections contain a first superconducting material, and have a first electric voltage insulation, the conductors of the radially outer sections comprising a second superconducting material and having a second electric voltage insulation, wherein the first superconducting material has a higher critical magnetic field than the second superconducting material and the first voltage insulation has, at least in certain areas, a smaller dielectric strength than the second voltage insulation, and with a shielding coil which is connected in series with the main field coil, radially surrounds the main field coil, and comprises conductors which contain the second superconducting material and which are surrounded by the second electric voltage insulation, wherein the main field coil and the shielding coil are superconductingly short-circuited during operation (persistent mode) and are protected by several ohmic resistances and/or diodes in case of breakdown of the superconductivity (quench).
U.S. Pat. No. 5,644,233 discloses a magnet configuration with an actively shielded magnet coil system comprising a radially inner main field coil and a radially outer shielding coil which are protected by resistances.
In particular, in RF applications of superconducting magnet coil systems, a considerable stray field is generated which represents a danger to the surroundings of the magnet. The term “stray” field thereby characterizes the remote field of the magnet coil system which is dominated by the dipole-like field portion. In actively shielded magnet coil systems, the stray field is tailored by a shielding coil which is wound in an opposite direction to that of the main field coil, i.e. with a dipole moment which is opposite to the main field coil, such that the overall dipole moment of the magnet coil system is approximately zero. In the event of a superconduction breakdown, it must be ensured that the spatially defined stray field does not exceed its limit value. The field at the location of the specified 5G line must e.g. be smaller than 5 Gauss even during a quench.
In magnet coil systems without quench protection, the stray field remains in case of a quench. High voltages are thereby generated which can damage the magnet coil system. For this reason, one tries to prevent any damage to the magnet coil system due to current changes and associated voltages during a quench.
Superconducting magnet coil systems therefore generally have a device which diverts the magnetic current from the coil sections, which have become resistive during a quench, via protective elements, e.g. resistors.
M. N. Wilson “Superconducting Magnets” (Oxford University Press) discloses bridging of coil sections of the magnet coil system with ohmic resistances. The magnet coil system is thereby divided into meshes which are connected in series, wherein each mesh comprises a coil section of the magnet coil system and a resistance, thereby generating partial areas with a smaller intrinsic inductance. The currents quickly decay in these partial regions, thereby preventing large voltages. However, in case of a quench, currents of different strengths flow in the different meshes of protective resistances and coil sections, which can produce unacceptable uncompensated excess stray fields.
EP 0 144 171 B1 discloses an actively shielded magnet coil system which comprises a resistance network as protection from excess voltages in case of a quench, wherein parts of the magnet coil system are bridged by ohmic resistances. However, the stray field assumes values which may exceed specifications.
U.S. Pat. No. 5,644,233 therefore proposes a magnet coil system, wherein the main field coil and the shielding coil are divided into an equal number of coil sections and are serially connected in pairs, forming one mesh together with an ohmic resistance, wherein each pair of coil sections of main field and shielding coil, which are connected in series within one mesh, has almost identical dipole moment but of different sign. Each mesh therefore comprises one coil section of the shielding coil and one coil section of the main field coil with similar but opposite dipole moments, and a protective resistance which bridges the two coil sections. In the event of a quench in such a section, the stray field only slightly changes since each mesh is approximately free from dipole moments and only minimally contributes to the stray field irrespective of the current that flows in this mesh. The disadvantage of this method is the large constructive and technical expense for the production of the paired wiring of radially distant coil sections of the magnet coil system, since each coil section of the radially inner main field coil must be wired to one coil section of the radially further outward shielding coil.
U.S. Pat. No. 6,563,316 proposes a magnet configuration, wherein not all of the sections which are bridged by a protective element have a negligible dipole moment. This configuration nevertheless considerably reduces the danger of an excess stray field in the event of a magnet configuration quench. The conventional configuration has an additional, enclosed current path which has a non-vanishing number of windings per unit area and is inductively coupled to at least one coil section. This method is also disadvantageous due to the large constructive and manufacturing expense of paired wiring of radially distant coil sections of the magnet coil system.
It is the underlying purpose of the present invention to propose a magnet coil system which realizes effective quench protection without excess stray fields but with little constructive expense.